The occupant detection system detects a sitting condition such as existence or nonexistence of an occupant sitting on a seat which is provided in a vehicle, weight or the like. The detected sitting condition is used, for example, for operation control of an air bag device of the vehicle and flashing control of an indicator lamp thereof. The occupant detection system determines the sitting condition of the occupant by a load sensor which measures the weight of the occupant sitting on the seat and by using load data acquired from the load sensor. With respect to a strain gauge used for the load sensor there may be cases where the load outputted fluctuates due to temperature variation. Therefore, a temperature sensor is integrated with the load sensor so as to be located in the vicinity thereof, so that the fluctuation of the load outputted by the load sensor may be corrected based on the temperature detected by the temperature sensor.